The present invention relates to a method of ultrasonic-mounting an electronic component and a ultrasonic mounting machine.
When a semiconductor chip, which is an example of the electronic component, is mounted on a circuit board by flip chip bonding, bumps of the semiconductor chip are headed for the circuit board, then the bumps, which are made of gold or solder, are respectively bonded to electrodes of the circuit board. A space between the semiconductor chip and the circuit board is filled with synthetic resin so as to protect a circuit face of the semiconductor chip, prevent the bumps from corrosion and improve bonding strength therebetween.
These days, the semiconductor chip is flip-chip-bonded by applying ultrasonic vibrations. By applying ultrasonic vibrations, bonding sections between the bumps and the electrodes are alloyed and bonded.
A method of flip-chip-bonding a semiconductor chip by applying ultrasonic vibrations is shown in FIG. 8. The semiconductor chip 10 is air-sucked and held by a boding tool 40 and corresponded to a circuit board 20 supported by a supporting stage 30. Ultrasonic vibrations, which are generated by an ultrasonic transducer 49 attached to an end of the bonding tool 40, are applied to the semiconductor chip 10, so that the semiconductor chip 10 can be bonded to the circuit board 20.
FIG. 9 is an enlarged view of bonding the semiconductor chip 10 to the circuit board 20 by applying ultrasonic vibrations to the semiconductor chip 10. Ultrasonic vibrations are applied in a direction parallel to a surface of the semiconductor chip 10, so that the semiconductor chip 10 is horizontally vibrated with respect to the circuit board 20 supported by the supporting stage 30. By vibrating the semiconductor chip 10, bumps 12 of the semiconductor chip 10 and electrodes of the circuit board 20 are mutually grinded, so that they are bonded each other.
Generally, the semiconductor chip 10 is vibrated in a direction parallel to the surface thereof, but the semiconductor chip may be vertically vibrated with respect to the surface thereof (see Japanese Patent Gazette No. 2001-57376).
Applied energy for ultrasonic bonding is generally indicated by the following formula:
  E  =            ∫      0              t        0              ⁢          μ      ⁢                          ⁢              L        ·        2            ⁢                          ⁢      π      ⁢                          ⁢      fA      ⁢                          ⁢      sin      ⁢                          ⁢              (                  2          ⁢                                          ⁢          π          ⁢                                          ⁢          ft                )            ⁢              ⅆ        t            wherein E (μJ/m2) is the applied energy, μ is a frictional coefficient between the bumps and pads, L (Pa) is a load, f (Hz) is frequency, A (μm) is amplitude, and t (sec.) is time. According to the formula, the applied energy for bonding is increased with increasing the load.
FIGS. 10A and 10B explain action of the bump 12 when ultrasonic vibrations are applied to the semiconductor chip 10 in the direction parallel to the surface thereof. In FIG. 10A, displacement of the bump 12 in the direction of the vibrations is zero; in FIG. 10B, the displacement of the bump 12 in the direction of the vibrations is maximized. When ultrasonic vibrations are applied to the semiconductor chip 10, the bump 12 is slightly inclined rightward and leftward. At that time, stress working on the bump 12 concentrates to an outer circumferential part A of the bump 12.
FIG. 11 shows a result of simulation of working the stress on the bump 12 bonded by applying ultrasonic vibrations. According to FIG. 11, the stress concentrates to the outer circumferential part when the displacement of the bump 12 is maximized.
When the displacement of the bump 12 is maximized, the stress concentrates to the outer circumferential part of the bump 12 so that metal bonding in the outer circumferential part is promoted, but metal bonding in a center part of the bump 12 is insufficient. Thus, in a contact area of the bump 12 and the electrode 22, the both cannot bonded uniformly. Further, voids are formed in a bonding section of the bump 12, so that bonding strength or bonding reliability must be lowered. In FIG. 12, voids B are formed close to the outer circumferential part of the bump 12.
Note that, in the method of vertically applying ultrasonic vibrations to the electronic component, if the electronic component is mounted onto a relatively soft substrate, e.g., plastic substrate, it is difficult to correctly apply energy to a bonding section, so that the electronic component cannot be mounted with prescribed bonding strength.